A hot dip galvannealed steel sheet is excellent in weldability and paintability as compared with a galvanized steel sheet which has not been subjected to alloying treatment, and thus is widely utilized in various fields, in particular, in application to car bodies. A hot dip galvannealed steel sheet for an application is press formed and used. However, a hot dip galvannealed steel sheet is disadvantageous in that a hot dip galvannealed steel sheet is inferior to a cold rolled steel sheet in press-forming properties. This is because the sliding resistance of a hot dip galvannealed steel sheet in a press die is high as compared with that of a cold rolled steel sheet. More specifically, it becomes difficult for the hot dip galvannealed steel sheet to flow into the press die at a portion where the sliding resistance between the die and a bead is high, resulting in the fact that the steel sheet is likely to break.
A hot dip galvannealed steel sheet refers to a sheet formed by galvanizing a steel sheet and then heating, and thus Fe in the steel sheet and Zn in a plated layer are dispersed to cause alloying reaction, whereby an Fe—Zn alloy phase is formed. The Fe—Zn alloy phase is a film generally containing a Γ phase, a δ1 phase, and a ζ phase. There is a tendency that the hardness and the melting point decrease with a decrease in the Fe concentration in the order of the Γ phase, the δ1 phase, and the ζ phase. Therefore, from the viewpoint of the sliding performance, a film having a high hardness and a high Fe concentration, for which a melting point is high and adhesion is likely to occur, is effective. The hot dip galvannealed steel sheet in which press-forming properties are regarded as important properties is manufactured in such a manner that the average Fe concentration of the film is high.
However, such a film having a high Fe concentration has problems in that the F phase, which is hard and vulnerable, is likely to be formed on the plating-steel sheet interface, and a phenomenon in which the film is separated from the interface, i.e., a so-called powdering, is likely to occur during processing. Therefore, in order to achieve both the sliding performance and powdering resistance, a method is employed which provides a hard Fe alloy as a second layer to an upper layer by electroplating or the like as disclosed in Patent Document 1.
As a method of improving the press-forming properties during the use of a galvanized steel sheet, a method of applying a lubricating oil having a high viscosity is widely used in addition to the above method. However, this method has problems in that coating defects may be caused by insufficient degreasing due to the high viscosity of a lubricating oil used in a coating process, the press performance becomes unstable due to the shortage of oil at the time of pressing, etc. Therefore, improvement of the press-forming properties of hot dip galvannealed steel sheets themselves has been strongly demanded.
As a method of solving the above-mentioned problems, Patent Documents 2 and 3 disclose techniques of subjecting the surface of a zinc steel sheet to electrolytic treatment, immersion treatment, coating oxidation treatment, or heat treatment to form an oxide film containing ZnO as a main component, to thereby improve the weldability and the processability.
Patent Document 4 discloses a technique of immersing a galvanized steel sheet in an aqueous solution containing 5 to 60 g/l of sodium phosphate and having a pH of 2 to 6, subjecting the surface of the plated steel sheet to electrolytic treatment, or applying the aqueous solution to the surface of the galvanized steel sheet to form an oxide film containing a P oxide as a main component on the surface of the galvanized steel sheet, to thereby improve press-forming properties and chemical conversion properties.
Patent Document 5 discloses a technique of subjecting the surface of a galvanized steel sheet to electrolytic treatment, immersion treatment, coating treatment, coating oxidation treatment, or heat treatment to form Ni oxide thereon, to thereby improve press-forming properties and chemical conversion properties.
Patent Document 6 discloses a technique of bringing a hot dip galvannealed steel sheet into contact with an acid solution to form an oxide containing Zn as a main component on the surface of the steel sheet, to thereby suppress the adhesion between a plated layer and a press die and improve the sliding performance.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 1-319661    Patent Document 2: Japanese Unexamined Patent Application Publication No. 53-60332    Patent Document 3: Japanese Unexamined Patent Application Publication No. 2-190483    Patent Document 4: Japanese Unexamined Patent Application Publication No. 4-88196    Patent Document 5: Japanese Unexamined Patent Application Publication No. 3-191093    Patent Document 6: Japanese Unexamined Patent Application Publication No. 2002-116026
Technologies disclosed in Patent Documents 1 to 6 are effective for forming a hot dip galvannealed steel sheet having a relatively low hardness which is frequently used for automobile exterior panels. However, in a high-strength hot dip galvannealed steel sheet in which the contact pressure with a die increases due to high load at the time of press forming, an effect of improving the press-forming properties cannot be necessarily obtained stably.